Slag which is sometimes used for the production of fibrous insulating material is a waste product from blast furnace operations, and can for example contain 39% CaO, 37% SiO.sub.2, 18% Al.sub.2 O.sub.3, 4% MgO, and some small quantity of Fe.sub.2 O.sub.3 besides traces of other materials. Such material is somewhat similar to material from which the refractory bricks are made, and will actually dissolve or wash away the refractory bricks if the temperature is sufficient to ensure that the slag is free running. This is one of two reasons why electric furnaces are normally not used in the melting of slag before the slag is poured onto a spinning wheel for production of fibrous insulating material. The other reason is the difficulty of preventing clogging of an outlet, and an invention to overcome that other difficulty is the subject of a companion application.
Because of those two difficulties, slag is usually heated in a coke fired furnace, and as the slag melts it percolates through the coke bed and runs out an aperture in the base of the furnace.
The temperature at which the furnace runs can be as much as 1,600.degree. C. (usually between 1,450.degree. and 1,500.degree. C.) and at this temperature, the steel walls of the furnace can be seriously damaged. Consequently, it is usual practice in a coke fired furnace to surround the furnace with a jacket through which water continually passes, and the water has the effect of chilling the slag which forms immediately adjacent the inner wall of the surface, so that the slag itself solidifies and forms its own refractory insulation. With age and the continual use, the steel wall of the furnace gradually deteriorates, but when it deteriorates to the point where water from the surrounding jacket can enter the furnace, the water will enter a furnace containing a relatively small amount of molten slag, and a relatively large amount of burning coke, the water then merely tending to quench combustion and chilling the contents of the furnace, and those contents must be then separately removed and the walls separately replaced.
This arrangement however, is entirely unsatisfactory for an electric furnace. If a wall of an electric furnace deteriorates to the point where water from a surrounding jacket can enter the electric furnace, explosion is likely to occur because of the relatively large amount of molten slag within that furnace, and the absence of large amounts of coke. For this reason, heretofore electric furnaces used in the fusing of slag have utilised linings of refractory bricks which are frequently replaced.
In the U.S. Pat. No. 1,946,083 of LAMBIE there was described a flux wall having tuck stones arranged to be air-cooled, in a gas fired glass furnace. The U.S. Pat. No. 2,042,660 to HULTON also described an air cooling arrangement for furnace walls.
U.S. Pat. No. 2,686,821 to McMULLEN described an electric furnace for melting inorganic refractory oxidic material which utilised a water cooling jacket. The danger of explosion referred to above does not exist in this arrangement, however, since the melt is confined to the upper portion of the container, and is decanted by tilting the container.
U.S. Pat. No. 2,790,019 STALEGO explains the difficulty encountered in the freezing of the melt adjacent a pouring spout, and uses the overflow principle in a refractory lined furnace.
Although BERCZYNSKI described in his U.S. Pat. No. 3,612,501 an annular water-cooling jacket, this was used for water-cooling a refractory lining, and, as said above, such linings are unsatisfactory in the fusing of metal oxides.
The main object of this invention is to provide improvements whereby an electric furnace can be used for production of fused oxide compounds for example, such as are used in the production of mineral fibre insulating batts, and can be used for a continuous production for a much longer period of time than in the "overflow" type furnaces of McMULLEN or STALEGO aforesaid.